As glasses which are used for windows of vehicles such as automobiles, electric trains, etc. and buildings, a laminated glass having a function to shield heat rays is investigated, and a part thereof has already been put into practical use. In recent years, such a laminated glass gets attention from the viewpoint of energy conservation because it prevents incidence of heat rays.
This laminated glass transmits a visible light of whole rays and selectively reflects or absorbs heat rays. For example, when this is used for a windowpane, in a season in which the sunlight is strong, it can suppress a temperature increase in the interior of a room to be caused due to incidence of heat rays, whereas in a season in which the sunlight is weak, and the heating is used, it can suppress an escape of heat from the interior of a room to the outdoors. For that reason, the use of this laminated glass can greatly enhance the utilization efficiency of energy and is useful for energy conservation.
This laminated glass can be obtained by laminating a heat ray shielding film on a glass.
Patent Literature 1 discloses an optical interference film which reflects an infrared light while transmitting a visible light, comprising multiple alternating layers of at least three kinds of layers each having an optical thickness ranging from 0.09 to 0.45 μm, wherein a refractive index of a polymer of the second layer is intermediate between a refractive index of a polymer of the first layer and a refractive index of a polymer of the third layer. Patent Literature 1 discloses that one of the three layers may be made of polyethylene-2,6-naphthalenedicarboxylate. However, Patent Literature 1 does not investigate a laminated polyester film having a layer structure in which two kinds of layers are alternately laminated, the laminated polyester film being excellent in not only near-infrared light shielding properties but processability into a laminated glass.
Patent Literature 2 describes a birefringent dielectric multilayer film which reflects at least 50% of light in a band having a width of at least 100 nm in a wavelength region of interest. Patent Literature 2 describes that the multilayer film includes alternating layers of a first polymer and a second polymer and may be laminated on a non-planar glass layer and used in vehicular windshields. Meanwhile, in the polymers of the respective layers specifically investigated in Patent Literature 2, the first polymer was a polyester, whereas the second polymer was PMMA.
Patent Literature 3 discloses a transparent multilayer device which reflects an infrared light while transmitting a visible light, comprising a polymer film including a plurality of layers and a transparent conductor having at least one layer containing a metal or a metal compound. In Patent Literature 3, as for the case of mirrors, namely, combinations of the multilayer films obtained by stretching in two directions within the film plane, there are exemplified PEN/Ecdel (a trade name for thermoplastic elastomer), PEN/sPS, PEN/copolymerized PET, and the like. Patent Literature 3 describes that the transparent multilayer device may be used in windshields for automobiles or the like, or windowpanes. However, Patent Literature 3 does not investigate an increase of the processability into a laminated glass.
In addition, Patent Literature 4 discloses a near heat ray shielding film in which a first layer having a melting point ranging from 250 to 260° C. and comprising ethylene terephthalate as a main recurring unit and a second layer having a melting point ranging from 200 to 245° C. and comprising ethylene terephthalate as a main recurring unit are alternately laminated and describes that an interlayer separation phenomenon is inhibited by increasing adhesion between layers by using resins having a composition close to each other in the first layer and the second layer. However, in the resins having a composition close to each other, since a difference in refractive index between the layers is small, the layer number is required to be increased, and it is necessary to take a complicated layer structure in which a first laminated portion and a second laminated portion having a different layer thickness ratio from the first layer and the second layer, respectively are further laminated. In addition, Patent Literature 4 does not investigate an increase of the processability into a laminated glass.
Patent Literature 5 discloses a biaxially stretched polyester film suitable as an intermediate film for a laminated glass with a good appearance, comprising an alternate laminate of two kinds of resins and having excellent glass scattering prevention and crime prevention performance. In Patent Literature 5, for the purpose of increasing the crime prevention performance, the Young's modulus of the film at room temperature is specified. However, Patent Literature 5 is concerned with a polyester film having an excellent crime prevention performance, but it does not make investigations while paying attention to an increase of near-infrared light shielding properties and processability into a laminated glass.
In the light of the above, in the case of an alternate laminate made of polymers of different kinds which has been conventionally investigated, for example, PMMA has a high hardness, and therefore, when processed into a laminated glass, a phenomenon of generation of fine irregularities is not caused. Thus, such was not acknowledged as a problem. However, it has been newly clarified that in the process of proceeding with investigation of films for laminated glass using an alternate laminate made of polyesters each other for the purposes of recycle properties, solving of separation between layers, and so on, when a polyester-based alternate laminate having excellent near-infrared light shielding properties is processed into a laminated glass, fine irregularities are generated on the film surface. Furthermore, it is the present state that a film having increased processability into a laminated glass is demanded.    Patent Literature 1: JP-A-4-313704    Patent Literature 2: JP-A-2011-225445    Patent Literature 3: JP-T-11-508380    Patent Literature 4: WO 2005/040868A    Patent Literature 5: JP-A-2005-186613